Solar energy collectors which currently are available rely on the absorption principle and they are provided with a matt black surface coating which serves to absorb incident energy and to transfer such energy to a fluid medium which is passed through the collectors. Simple such absorbing surfaces are characterised by poor efficiency (low absorptivity and high emissivity) and attempts have been made to increase heat collection efficiency by use of so-called solar selective surface coatings. Such coatings provide for high absorption of incident solar radiation and low emittance of infra-red radiation.
U.S. Pat. No. 2,917,817 discloses a solar collector or, more specifically, a so-called receiver which employs what might be regarded as a rudimentary solar selective surface coating. The coating is achieved by electrolytically depositing a dark metal surface layer onto a bright metal base or by oxidising the outer surface of the bright metal base so as to obtain a surface layer which is both light absorbing and transparent to heat radiation. The bright metal base is constituted by the receiver itself, the receiver being employed to transfer collected heat to water or another heat exchange medium, and hence the referenced patent does not teach the application of a two-layer coating to a carrier for a fluid medium.
U.S. Pat. No. 3,952,724 does teach a solar energy collector arrangement in which glass tubes are employed as carriers for heat exchange fluid, typically water, and such tubes are provided with an energy absorbing coating which is specified to have a high absorptivity and low emissivity. However, this patent also does not specify a two-layer coating, the exemplified coating materials being black chrome, nickel, lampblack, carbon or copper. The selected coating material is compounded for application to the tube surface, such as by painting.
Of further interest in the context of the present invention is a report made in publication RI8167 of the Bureau of Mines Reports of Investigations, 1976, entitled "Reflectance and Emittance of Spectrally Selective Titanium and Zirconium Nitrides". Such report considers the applicability of titanium and zirconium nitride films as solar selective coatings for solar collectors. However titanium and zirconium metals are expensive and for this and other reasons pertaining to the deposition of nitride films, solar collectors employing such film materials do not lend themselves to commercially viable production.
Other solar selective surface coatings which have been proposed are metal-oxides, but such coatings are not very stable at high temperatures in vacuum, such conditions being the normal conditions to which the coatings are exposed in practice.